The present invention relates to a method and a system for terminating inverter operation of a converting device having a bridge connection of thyristor valves and connected to a DC power transmission system, particularly a DC power transmission system having more than two terminals.
FIG. 1 shows an example of a multi-terminal DC power transmission system, i.e., a DC power transmission system having more than two terminals. In the illustrated example, there are four terminals or converting stations 8a, 8b, 8c, 8d. The converting stations 8a-8d are interconnected by a pair of DC lines 7a, 7b forming a DC power transmission system, and are respectively connected to AC power transmission systems 1a, 1b, 1c, 1d. Each converting station comprises a converting device 4a, 4b, 4c, 4d. The AC terminals of each converting device are connected to the AC power transmission system 1a, 1b, 1c, 1d through a transformer 3a, 3b, 3c, 3d and an AC circuit breaker 2a, 2b, 2c, 2d. The DC terminals of each converting device are connected through a reactor 5a, 5b, 5c, 5d to the DC lines 7a and 7b.
Each of the converting devices 4a-4d comprises, as shown in FIG. 2, a bridge connection oil thyristor valves 6u, 6v, 6w, 6x, 6y, 6z. The valves 6u, 6v, 6w have one end connected to a negative line 7b, so that they are called valves of lower potential side. The valves 6x, 6y, 6z have one end connected (through a reactor 5a, 5b, 5c, 5d) to a positive line 7a so that they are called valves of higher potential side. FIG. 2 also shows AC terminals R, S, T for connecticn with three-phase AC power transmission system (1a-1d in FIG. 1), and DC terminals P, N for connection to the DC lines 7a, 7b through a reactor (indicated by 5 as representative of 5a-5d in FIG. 1). In FIG. 2, v.sub.d denotes DC voltage across the DC terminals P, N and i.sub.d denotes DC current through the DC terminals P, N.
Assume that the converting device in one of the converting stations, say 8d, has been in the inverter operation and it is desired that the inverter operation of the converting device be terminated. Conventionally, the DC current through the converting apparatus 4d is reduced to zero, and upon detection or confirmation of the current being zero, gate pulses are blocked (in other words the converting device is gate-blocked).
However, DC current id flowing through the converting device contains, as shown in FIG. 3, ripples because of the DC voltage v.sub.d of the converting device and the DC line voltage v.sub.dL beyond the reactor 5d, so that it is very difficult to detect the current being zero.
In addition, if the gate pulses are blocked while the DC current i.sub.d is still flowing, as shown in FIG. 3, the interruption of the current need to be completed during the period of t.sub.1 -t.sub.2 when the DC voltage v.sub.d is greater (in magnitude) than the line voltage v.sub.dL as shown by hatching in FIG. 3. After t.sub.2, v.sub.d becomes smaller than v.sub.dL. If the interruption is not completed by t.sub.2, a very large current, shown by dotted line, flows through the converting device whose gate pulses are blocked.
It is thus seen that it is vital to make sure that the DC current is zero prior to blocking the gate pulses for termination of the inverter operation of a converting device in a DC power transmission system. If the gate pulses are blocked while the DC current is still flowing, an excessive current flows, which may cause a serious breakdown necessitating interruption of the operation of the entire DC power transmission line.